Synthesis, characterization of novel cyclohexenone derivatives and computation of their optical response

Article abstract of DOI:10.1016/j.molstruc.2014.04.074

The present study reports the successful development of five new cyclohexenone derivatives (CDs) using rational design synthesis principles and quantum chemical calculations. These new CDs were synthesized by following a convenient route of Robinson annulation, and the molecular structure of these CDs were later confirmed by various analytical techniques such as ¹H NMR, ¹³C NMR, FT-IR, UV-Vis spectroscopy and Mass spectrometry. The results from spectroscopic studies show that the as-synthesized CDs molecules apparently emit violet light at about 406-414 nm. Moreover, polarizability (α) and first static hyperpolarizability (β) were computed by density functional theory (DFT). In addition, the UV-Vis spectra, transition character and electronic structures of these CDs were computed by using the time dependent density functional theory (TD-DFT). It was interesting to note that the values of computed and experimental electronic transitions (λ_max) were in good agreement. Finally, the measurements of higher non-linear optical (NLO) response of our newly synthesized CDs suggest their potential for application in photonic devices.

Full text of DOI:10.1016/j.molstruc.2014.04.074

Accepted Manuscript
Synthesis, Characterization of Novel Cyclohexenone Derivatives and Compu-
tation of their Optical Response
Amir Badshah, Muhammad Faizan Nazar, Asif Mahmood, Waqas Ahmed,
Muhammad Imran Abdullah, Muhammad Naveed Zafar, Usman Ali Rana
PII:
S0022-2860(14)00441-4
DOI:
http://dx.doi.org/10.1016/j.molstruc.2014.04.074
MOLSTR 20581
Reference:
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24 April 2014
24 April 2014
Please cite this article as: A. Badshah, M.F. Nazar, A. Mahmood, W. Ahmed, M.I. Abdullah, M.N. Zafar, U.A.
Rana, Synthesis, Characterization of Novel Cyclohexenone Derivatives and Computation of their Optical Response,
Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.04.074
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Synthesis, Characterization of Novel Cyclohexenone Derivatives and
Computation of their Optical Response
Amir Badshaha, Muhammad Faizan Nazarb*, Asif Mahmoodc, Waqas Ahmedd,
Muhammad Imran Abdullahe, Muhammad Naveed Zafarb, Usman Ali Ranaf
aInstitute of Chemical Sciences, University of Peshawar, Pakistan
bDepartment of Chemistry, University of Gujrat, Gujrat, Pakistan
cDepartment of Chemistry, University of Sargodha, Sargodha, Pakistan
dOffice of Research, Innovation and Commercialization, University of Gujrat, Pakistan
eInstitute of Chemistry, University of the Punjab, Lahore, Pakistan
fDeanship of Scientific Research, College of Engineering, King Saud University, Riyadh,
Saudi Arabia
*To whom correspondence should be addressed:
Name:
Dr. Muhammad Faizan Nazar
E-mails:
faizan_qau@yahoo.com, faizan.nazar@uog.edu.pk
Office # 12, Department of Chemistry, Institute of
Chemical and Biological Sciences, University of Gujrat,
Gujrat-50700, Pakistan
Postal address:
Phone number:
Fax number:
+923016942411
+92533643167
1

Synthesis, Characterization of Novel Cyclohexenone Derivatives and
Computation of their Optical Response
Abstract
The present study reports the successful development of five new cyclohexenone
derivatives (CDs) using rational design synthesis principles and quantum chemical
calculations. These new CDs were synthesized by following a convenient route of
Robinson annulation, and the molecular structure of these CDs were later confirmed by
1
various analytical techniques such as H NMR, 13C NMR, FT-IR, UV–Vis spectroscopy
and Mass spectrometry. The results from spectroscopic studies show that the as-
synthesized CDs molecules apparently emit violet light at about 406-414 nm. Moreover,
polarizability (α) and first static hyperpolarizability (β) were computed by density
functional theory (DFT). In addition, the UV–Vis spectra, transition character and
electronic structures of these CDs were computed by using the time dependent density
functional theory (TD-DFT). It was interesting to note that the values of computed and
experimental electronic transitions (λmax) were in good agreement. Finally, the
measurements of higher non-linear optical (NLO) response of our newly synthesized
CDs suggest their potential for application in photonic devices.
Keywords: Cyclohexenones; Fluorescence; Polarizability; Hyperpolarizability; TD-
DFT; NLO.
1. Introduction
At present, the developments of materials that exhibit second-order nonlinear optical
(NLO) responses are receiving significant attention, primarily due the potential
applications of these materials in the area of optoelectronic technology [1]. On the basis
of theoretical and experimental investigations, several principles have been suggested to
improve the second-order NLO response. These principles include planar donor-π-
conjugated bridge–acceptor (D– π–A) type [2], bond length alternation (BLA) theory [3],
auxiliary donor and acceptor representation of heterocycles [4], and twisted π-electron
systems [5]. Literature suggests that the large second-order NLO response can be
attained by optimizing the D/A strength and/or by extending the conjugated bridge [6].
Owing to the flexibility of altering the molecular structure, the organic molecules are
2

therefore investigated as the potential candidates for optimizing the NLO properties in
optical signal processing, data storage, sensor protection and imaging [7,8].
Among a variety of organic molecular systems, the chalcones are open chain flavonoids
with two aromatic rings attached by an unsaturated carbonyl moiety [9-11]. Though, the
chalcones are notorious due to their biological properties, in several cases, the presence
of extensive conjugation in these molecules also provides charge transfer axis. In
addition, one can easily incorporate the appropriate functionalities on the two aromatic
rings of the chalcones and thereby can tune one aromatic ring as donor and second as
acceptor [12,13]. This charge transfer from donor to acceptor can take place through
charge transfer axis [14]. Since, they display NLO responses, chalcones with delocalized
π-electron systems are superior candidates as NLO materials compared to the standard p-
nitroaniline [15].
A number of studies show that the cyclic chalcones display different types of
pharmacological activities [16,17]. Michael additions of ethyl acetoacetate to numerous
chalcones, followed by internal aldol condensation, have compelled them as effective
synthons in various projected synthesis of azoles and cyclohexenones derivatives [18-
22]. Extensive optical properties and various chemical functions of cyclohexenone
derivatives also make them valuable compounds for applications such as organic light-
emitting devices (light emitting laser, laser pointers, light-emitting diodes etc.) [23] and
fluorescent probes for membranes [24,25].
This work demonstrates the synthesis and characterization of some novel cyclohexenone
derivatives (CDs) via a convenient route of Robinson annulations. In order to fully
explore the properties of these newly synthesized CDs, non-linear optical properties and
absorption spectra of these compounds were calculated using quantum chemical
methods. Our results revealed that the investigated molecules are likely to have
interesting NLO properties as their hyperpolarizability values etc. are high. Moreover, in
the present work, we were also able to generate some high responsive optical moieties in
our newly synthesized CDs that may possibly lead to the molecular and material
engineering pathways in future.
2. Materials and Methods
2.1. Reagents
3

The reagents; ethyl acetoacetate, 4-Chlorobenzaldehyde, 2-Methoxybenzaldehyde, 4-
Methoxybenzaldehyde, 2-Furfural and 2-Acetylfuran were purchased from Fluka
(Germany). The liquid reagents were distilled at their boiling points and the solid
reagents were characterized by recording their melting points. No further purification
was required. Sulphuric acid and hydrochloric acid (37%) were obtained from Stedee
Ltd. The solvents chloroform, ethyl acetate, absolute ethanol and pet ether were
purchased from Sigma Aldrich (Germany). All the solvents were used after necessary
purification and drying according to the standard procedures. The dried solvents were
stored over molecular sieves (4Å).
2.2. Compound Characterization Techniques
Rf values were calculated by using precoated silica gel aluminum backed plates Kiesel
gel 60F254 Merck (Germany) using ethylacetate : pet-ether (1:4) as developing solvents.
Melting points of the compounds were determined in open capillaries using Gallenkamp
melting point apparatus and are uncorrected. The FT-IR spectral data were recorded on
1
Bio-Rad Merlin Spectrophotometer using KBr discs. H NMR and 13C NMR spectra
were recorded on Bruker (300 MHz) AM-250 spectrometer in CDCl3 solution using
TMS as internal standard. EIMS was recorded on Agilent mass spectrometer. Purity of
each compound was ascertained by thin layer chromatography. The purification of
synthesized compounds was achieved mostly through recrystallization, the use of solvent
extraction, or by making preparative thin layer chromatography or column
chromatography whenever required.
2.3. Steady-State Fluorescence Measurements
Steady-state fluorescence (SSF) was performed on a Perkin Elmer LS 55 Luminescence
Spectrometer with PC controlled software FinWinLab. The Photo Multiplier tube (PMT)
voltage was kept at 665V. The emission and excitation slits were fixed at 6.0 nm each.
The excitation wavelength was taken as 331 nm. The scan range used was from 350-600
nm. Polarizers were kept clear and no cutoff was operating during the scan. The
temperature of the cell was fixed with the help of an external circulator water bath.
2.4. Spectrophotometric (UV-Visible) Measurements
4

Spectrophotometric measurements were performed on a Perkin-Elmer Lambda 20
ultraviolet-visible spectrophotometer with 1.0 cm quartz cells at a temperature of
25.0 0.1°C. The scan range used was from 200-400 nm.
2.5. General Synthetic Methods
The synthetic strategy for the titled compounds is outlined in the following Scheme 1.
Furanyl-containing chalcone analog 1 (3 mmol) and ethyl acetoacetate 2 (0.39 g, 0.40
mL, 3 mmol) were refluxed for 2 h in 10-15 mL ethanol in the presence of 0.5 mL 10%
NaOH aqueous solution. The reaction mixture was then poured with good stirring into
200 mL ice-cold water and kept at room temperature until the reaction product separated
as a solid, which was filtered off and recrystallized from ethanol.
1
The molar ratio, physical, FTIR, H NMR, 13C NMR and EIMS data for these
compounds are given below:
Ethyl-6-(4-methoxyphenyl)-4-(furan-2-yl)-2-oxocyclohex-3-ene-1-carboxylate
1
(1FuM4): Carmel solid, Yield 3.0 g (65%); m.p.: 83–85 °C. H NMR (CDCl3, 300
MHz): δ 1.07 (t, J=6.9 Hz, 3H), 4.06 (q, J=6.9 Hz, 2H), 3.00 (dd, J1=2.5 Hz, J2=16.5,
1H), 2.80–2.86 (m, 1H), 3.72 (dd J1= 2.7 Hz, J2=6.75, 2H), 6.60 (s, 1H), 7.57 (d, J=1.7
Hz, 1H), 6.53 (dd J1=1.8 Hz, J2=3.4, 1H), 6.76 (d, J=3.6 Hz, 1H), 7.24 (dd J1=2.1 Hz,
J2=6.6, 2H), 6.89 (dd J1=2.1 Hz, J2=6.6, 2H), 3.81 (s, 3H). 13C NMR (CDCl3, 300 MHz):
δ 193.42, 168.71, 151.09, 145.84, 145.69, 138.45, 134.00, 128.99, 127.94, 126.90,
126.85, 126.09, 114.00, 112.49, 61.16, 60.25, 58.41, 42.90, 31.98, 14.06. IR (KBr, cm-1):
1075, 1545, 1607, 1659, 1736, 2978. MS (EI): m/z (%) = 268(30), 134(100), 106(5),
51(4), 39(3), 240(10), 209(1), 119(10), 91(9), 65(5).
Ethyl-6-(2-methoxyphenyl)-4-(furan-2-yl)-2-oxocyclohex-3-ene-1-carboxylate
1
(1FuM2): Orange solid, Yield 2.9 g (64%); m.p.: 99–101 °C. H NMR (CDCl3, 300
MHz): δ 1.07 (t, J=6.9 Hz, 3H), 4.09 (q, J=6.9 Hz, 2H), 3.00 (dd, J1=2.5 Hz, J2=16.5,
1H), 2.80–2.86 (m, 1H), 3.72 (dd J1= 2.7 Hz, J2=6.75, 2H), 6.60 (s, 1H), 7.57 (d, J=1.5
Hz, 1H), 6.53 (dd J1=1.8 Hz, J2=3.2, 1H), 6.76 (d, J=3.6 Hz, 1H), 7.0–7.3 (m, 4H), 3.81
(s, 3H). 13C NMR (CDCl3, 300 MHz): δ 193.40, 169.14, 151.35, 145.36, 145.24, 139.75,
132.02, 130.25, 129.41, 129.37, 127.71, 127.05, 113.45, 112.51, 62.45, 60.61, 58.04,
43.02, 33.45, 14.03. IR (KBr, cm-1): 1070, 1520, 1599, 1655, 1735, 2982. MS (EI): m/z
5

(%) = 268(90), 134(100), 106(20), 51(10), 39(8), 240(13), 209(5), 119(45), 91(40),
65(10).
Ethyl-6-phenyl-4-(furan-2-yl)-2-oxocyclohex-3-ene-1-carboxylate (1FuO): Dull yellow
1
solid, Yield 3.0 g (65%); m.p.: 83–84 °C. H NMR (CDCl3, 300 MHz): δ 1.06 (t, J=7.2
Hz, 3H), 4.06 (q, J=7.2 Hz, 2H), 3.00 (dd, J1=2.3 Hz, J2=16.6, 1H), 2.80–2.86 (m, 1H),
3.77 (dd J1= 3.0 Hz, J2=6.9, 2H), 6.61 (s, 1H), 7.57 (d, J=1.5 Hz, 1H), 6.53 (dd J1=1.8
Hz, J2=3.2, 1H), 6.76 (d, J=3.9 Hz, 1H), 7.3 (m, 5H). 13C NMR (CDCl3, 300 MHz): δ
193.87, 169.36, 151.46, 146.09, 145.64, 142.85, 132.99, 129.00, 128.29, 128.29, 126.00,
126.00, 113.49, 112.60, 60.95, 60.26, 42.94, 33.38, 14.03. IR (KBr, cm-1): 1551, 1601,
1651, 1733, 2984. MS (EI): m/z (%) = 238(55), 134(100), 106(15), 51(7), 39(5), 210(20).
Ethyl-4,6-(furan-2-yl)-2-oxocyclohex-3-ene-1-carboxylate (1FuF): Red solid, Yield 3.2
g (61%); m.p.: 60–61 °C. 1H NMR (CDCl3, 300 MHz): δ 1.07 (t, J=7.1 Hz, 3H), 4.07 (q,
J=7.1 Hz, 2H), 3.00 (dd, J1=2.6 Hz, J2=17.0, 1H), 2.81–2.87 (m, 1H), 3.76 (dd J1= 2.9
Hz, J2=6.9, 2H), 6.60 (s, 1H), 7.59 (d, J=1.6 Hz, 1H), 6.53 (dd J1=1.8 Hz, J2=3.4, 1H),
6.76 (d, J=3.6 Hz, 1H), 5.5–6.09 (m, 3H). 13C NMR (CDCl3, 300 MHz): δ 193.79,
168.89, 153.49, 151.45, 146.09, 146.00, 145.61, 145.39, 133.49, 112.94, 112.65, 108.00,
60.94, 59.78, 42.94, 32.24, 14.03. IR (KBr, cm-1): 1503, 1600, 1672, 1735, 2975. MS
(EI): m/z (%) = 227(25), 134(100), 106(5), 51(3), 39(5).
Ethyl-6-(4-chlorophenyl)-4-(furan-2-yl)-2-oxocyclohex-3-ene-1-carboxylate (1FuC4):
1
Light brown solid, Yield 3.5 g (72%); m.p.: 96–98 °C. H NMR (CDCl3, 300 MHz): δ
1.08 (t, J=6.99 Hz, 3H), 4.08 (q, J=6.8 Hz, 2H), 3.05 (dd, J1=2.5 Hz, J2=16.5, 1H), 2.81–
2.90 (m, 1H), 3.72 (dd J1= 2.3 Hz, J2=6.9, 2H), 6.59 (s, 1H), 7.58 (d, J=1.8 Hz, 1H), 6.53
(dd J1=1.8 Hz, J2=3.6, 1H), 6.78 (d, J=3.6 Hz, 1H), 7.28 (dd J1=1.2 Hz, J2=4.5, 2H), 7.35
(dd J1=2.1 Hz, J2=6.6, 2H). 13C NMR (CDCl3, 300 MHz): δ 193.30, 169.08, 151.25,
145.78, 145.78, 139.41, 133.33, 129.03, 129.03, 128.75, 128.40, 128.20, 113.62, 112.66,
61.16, 59.73, 43.03, 32.99, 14.02. IR (KBr, cm-1): 752, 1554, 1605, 1656, 1732, 2985.
MS (EI): m/z (%) = 272(33), 134(100), 106(20), 51(6), 39(5), 244(6), 209(2).
On the basis of aforementioned results, the structures of cyclohexenone analogs are
shown in Table 1.
6

Table 1. Structures, Rf and λmax values for cyclohexenone derivatives.
aRf
Values
×100
UV
PL
Molecular
Formula
Molecular
Weight
Compound
Structure
λ
λ
max
max
(nm)
(nm)
O
O
O
1FuM4
C20H20O5
340.4
55
331
413
O
OCH3
O
O
O
1FuM2
C20H20O5
340.4
58
331
414
O
H3CO
O
O
O
1FuO
C19H18O4
310.0
63
332
413
O
O
O
O
1FuF
C17H16O5
300
59
59
332
331
406
413
O
O
O
O
O
1FuC4
C19H17ClO4
344.8
O
Cl
aSolvent for Rf values = Pet-ether:ethylacetae (4:1)
2.6. Computational Procedure
All computations data were collected using Gaussian 09 program package [26] and each
system was optimized using density functional theory. CAM-B3LYP functional [1(7)]
and 6-311g (d,p) basis set [1(8)] were used. Same of level of theory used to calculate the
non-linear optical properties.
7

The absorption spectra of all CDs were simulated by using TD-DFT calculations with
CAM-B3LYP. Ten (10) lowest singlet-singlet excitation energies were computed.
Symmetry constraints are not considered in all calculations.
Average polarizability (α) is determined by considering only the diagonal elements [27].
1/3 (
+
+
)
(1)
Ten components (βxxx, βxxy, βxyy, βyyy, β , β , β , β , β and β ) produced by
xxz
xyz
yyz
xzz
yzz
zzz
Gaussian 09W are used to calculate total hyperpolarizability (β ) [27]:
tot
(2)
3. Results and Discussion
3.1. Synthesis of Cyclohexenone
The reaction of chalcones and their heterocyclic analogs with ethyl acetoacetate in the
presence of basic condition underwent Michael addition followed by internal aldol
condensation to produce cyclohexenone, as outlined in Scheme 1.
Scheme 1. General methodology for the synthesis of Cyclohexenone derivatives.
The organic compounds were separated out by pouring the reaction mixture into water.
All compounds precipitated after being kept for several days. The solid products were
separated by filtration, dried and recrystallized from ethanol. High yields of the
8

cyclocondensation were obtained that varied from 61 to 72%. Structural analysis of the
newly synthesized cyclohexenone was carried out by analytical and spectral data. The IR
spectra of these compounds revealed a sharp strong absorption band around 1732-1736
cm-1 that can be correlated with the presence of the ester function in the structure of
cyclohexenone. Furthermore, another sharp strong absorption band at around 1651-1672
cm-1 assigned to the conjugated carbonyl group. The bands at 2975-2985 cm -1 accounted
for the OH group due to enol form and the hydrogen bonded carbonyl of ester group
appeared around 1500-1600 cm-1.
The 1H-NMR spectra substantiated the results of the IR analysis. One of the
representative compounds of cyclohexenone series, ethyl-6-(4-methoxyphenyl)-4-(furan-
2-yl)-2-oxocyclohex-3-ene-1-carboxylate (1FuM4), has been described here in detail
1
(Figure S1, Supporting Information shows an H-NMR spectrum and a summary of
integrals and peak positions of 1FuM4)
1
In the H-NMR spectrum of (1FuM4), the ethyl protons resonated as a triplet and a
quartet at 1.07 ppm 4.06 ppm integrating for three and two protons respectively. The
1
characteristic signal in the H NMR spectrum of compound (1FuM4) is however the
singlet of the vinylic proton (Ha) in the cyclohexenone ring, that resonated at
approximately 6.60 ppm integrating for one proton, and confirms that the intramolecular
cyclocondensation subsequent to the Michael addition actually took place. The signal
due to the methylene protons (Hd) appeared as two doublets at 3.00 ppm and at 3.07 ppm
respectively thereby indicating that they are diastereotropic protons. The signal due to Hc
appeared as a multiplet centred at 2.83 ppm integrating for one proton. The Hb proton
appeared as doublet at 3.72 ppm integrating for one proton. As for the protons in the
1
aromatic region, the HNMR spectrum allowed the assignments of the protons in the
furane ring at 6.53 ppm, 6.89 ppm and 7.24 ppm. The number of other aromatic protons
on the aryl moiety integrated in the 1H NMR spectra of compound (1FuM4) was in good
agreement with the factual one. The 13C NMR spectrum of 1FuM4 showed a
characteristic peak at 169.36 ppm, which confirms carbonyl carbon of the ester group. A
peak at 193.87 ppm further confirmed the presence of the carbonyl carbon of ketone.
While the peak at 132.99 ppm showed the presence of vinylic carbon. To get some idea
about the disposition of the constituent parts in the topical compounds, the single crystal
structure of one of the analogous compound in this series was reported earlier [28].
Our results revealed that the mass spectrum of (1FuM4) is also in accordance with the
proposed structure that was evidenced with the observation of characteristic peaks.
9

However, the molecular ion peak was found absent. The fragment peak at m/z 268 could
be ascribed to the loss of ester moiety, while the base peak appeared at m/z 134 could be
attributed to the result of Retro-Diels-Alder fission of cyclohexene ring. The peaks
appeared at m/z 240 and 209 are deemed to the loss of carbon monoxide and the side
chain respectively.
3.2. Absorption and Emission Spectra of Cyclohexenone Derivatives
The UV-Visible absorption spectra and fluorescence emission spectra of cyclohexenone
derivatives are shown in Figure 1.
1.0
0.8
0.6
0.4
0.2
0.0
500
400
300
200
100
0
emission
1FuM4
1FuM2
1FuO
1FuF
1FuC4
absorption
260
325
390
455
520
585
Wavelength (nm)
Figure 1. The normalized absorption and emission spectra of aqueous solution (3%
ethanol) of cyclohexenone derivatives.
The absorptions of cyclohexenone derivatives were located around 331 nm. Their
excitation wavelengths were all fixed at 330 nm. It is found that their intensity of
fluorescence differs from each other and their emission maximum (λ ) ranged from 406-
em
414 nm in the violet region.
10

3.3. Non-linear Optical Properties
Investigations of nonlinear optical properties are essential in designing materials for
signal processing, communication technology, optical memory devices and optical
switches. Hence, the values of average polarizability of cyclohexenone derivatives are
given in Table 2.
Table 2. Polarizablity (α) (1×10-24 esu) of synthesized cyclohexenone derivatives.
α
α
α
α
XX
YY
ZZ
Compound
1FuM4
284.97
270.94
342.48
257.96
279.14
251.31
242.60
293.05
205.50
244.32
143.18
150.90
183.53
106.86
135.53
270.90
261.12
315.56
245.78
285.34
1FuM2
1FuO
1FuF
1FuC4
Order of average polarizability of compounds was found to be: 1FuO >1FuC4 > 1FuM4 >
1FuM2 >1FuF. Polarizability of 1FuO was 315×10−24 esu. Moreover, the polarizability of
a molecule showed dependence on the energy gap between HOMO and LUMO. For a
molecule to be polarisable, a relatively small energy gap is usually required.
Additionally, literature suggests that a relatively large linear polarizability is required to
obtain large hyperpolarizabilities [29]. HOMO-LUMO compositions of synthesized
compounds are given in Figure 2.
11

12

13

Figure 2. HOMO and LUMO composition of synthesized cyclohexenone derivatives.
The static hyperpolarizabilities were calculated by using CAM-B3LYP method and the
computed β values of cyclohexenone derivatives are outlined in Table 3. These values
tot
are reported as 10-30 esu. The order β of four systems was: 1FuO > 1FuM2 > 1FuC4 >
tot
1FuM4 > 1FuF. Hyperpolarizabilities of all systems were found higher than the reported
values for p-nitroaniline [15].
Table 3. Hyperpolarizabilities β (1×10−33 esu) of synthesized cyclohexenone derivatives.
Compound
1FuM4
βXXX
βXXY
βXYY
βYYY
βtot
579.94
1323.14
1171.73
1102.96
-827.20
331.86
1170.62
312.23
899.34
1034.45
1FuM2
1FuO
1FuF
6639.06
-2317.92
61.95
3508.30
-897.83
1023.20
1042.95
149.16
542.28
-479.58
1172.30
5467.67
656.34
934.12
1FuC4
1035.72
3.4. Theoretical Optical Absorption Studies
In order to investigate the electronic transitions in all synthesized cyclohexenone
derivatives, the TD-DFT calculations were successfully carried out. Moreover, the
electronic spectra were simulated with TD-DFT/CAM-B3LYP method using optimized
geometries. In TD-DFT calculations, the first 10 spin-allowed singlet–singlet excitations
were calculated. The percentage contributions of molecular orbitals for bands formation
were obtained by using gaussum 2.2.4 Program [30]. Transition energy, maximum
wavelengths, oscillator strength, main configurations, and transition character are
presented in Table 4.
14

Table 4. TD-DFT calculation data of synthesized cyclohexenone derivatives.
λ
*
λ
max**
Egm
max
Compound
1FuM4
ƒgm
MO transition
H-2 L (70%)
H-2 L (69%)
H L (91%)
(nm)
(nm)
(eV)
331
319
3.09
2.08
1.73
0.732
0.700
0.690
1FuM2
331
332
316
321
1FuO
H-1 L (58%)
H L (87%)
1FuF
332
331
323
313
3.63
2.69
0.683
0.689
1FuC4
absorption maximum; * experimental value, ** computed value.
With the aim to fully explore the origin of the second-order NLO properties of the
synthesized compounds, a better elucidation of a structure–property relationship was
required. Hence, by using the sum-overstates (SOS) approach, a two-state model linked
between β and a low-lying charge-transfer transition has been recognized and reported
earlier [31]. In the static case, the following model expression was employed to estimate
β :
CT
(3)
In equation 3,
is the change of dipole moment between the ground and mth excited
state, fgm is the oscillator strength of the transition from the ground state (g) to the mth
excited state (m), and Egm is transition energy. In the framework of two-state model
expression, the hyperpolarizability is proportional to the product of the transition dipole
moments and oscillating strength and is inversely proportional to the cube of the
transition energy. Therefore, well-performing NLO material must possess a low-energy
CT excited state with large oscillator strength [32]. These factors (
, fgm, and Egm)
are closely related with each other and are governed by choice/strength of the
donor/acceptor along with the conjugated bridge. The most favorable combination of
these factors can provide a larger β value.
15

All compounds showed absorbance in the ultraviolet region. It is suggested that the
maximum absorption wavelength is an approximate measure of the transparency
achievable [33]. In the case of 1FuO and 1FuC4 systems, the HOMO to LUMO transition
is deemed most probable and of low energy. For all other systems, significantly low
transition energy was required (3.63-1.73 eV). Low transition energy results the large β
[34]. Our results show that the order of transition energy was opposite to that of the order
of hyperpolarizability. Moreover, the transition energy showed a decreasing order: 1FuF
> 1FuM4 > 1FuC4 >1FuM2 > 1FuO, whereas the NLO response showed an increasing
trend: 1FuF < 1FuM4 < 1FuC4 <1FuM2 < 1FuO.
Literature suggests that the oscillating strength (fgm) is directly related to the length of
conjugation [35]. Hence, the oscillating strength of all newly synthesized compounds
was recorded in the following order: 1FuM4 > 1FuM2 > 1FuO > 1FuF > 1FuC4. From
Tables 3 and 4, it can be clearly observed that the strong oscillating strength and small
transition energy were responsible for large β values. Here, the oscillator strength can
tot
be directly obtained from the TD-DFT calculations.
3.5. Density of States Calculation
In view of previously reported literature, the GaussSum was used for Density of states
(DOS) calculations [26]. DOS spectra provide the reliable information about band
structure. The results from Figure 3 show that the system with smallest band gap (Eg)
possesses maximal NLO response.
16

17

Figure 3. Density of states (DOS) spectra cyclohexenone derivatives.
Band gap is an important feature of the band structure; it strongly changes the optical and
electrical properties of the compounds. We also observed that since the DOS spectra are
qualitatively similar to NLO response of cyclohexenone derivatives, the study of band
structure could provide important information to explore the physical properties of our
newly synthesized compounds.
Here, we present the results of the band structure and the DOS for nonlinear optical
cyclohexenone derivatives. Hereby, the investigations of the electronic density of the
states for cyclohexenone derivatives have been performed for the first time. We found
that the system with smallest band gap (Eg) displays maximal NLO response (1FuO with
band gap 0.103 eV). We also observed that the transfer of electrons from one band to the
other occurred by means of carrier generation and recombination processes. Moreover,
the DOS spectra were found in good agreement with the NLO response of our
compounds, as the compound with smallest band gap possesses maximal NLO response.
Hence, the investigations of their band structure might play a critical role in
understanding of their physical properties (NLO). NLO response was found increasing as
follows: 1FuO >1FuM2 >1FuC4 > 1FuM4 > 1FuF.
4. Conclusions
The present study describes the synthesis and quantum chemical calculations of new
cyclohexenone derivatives (CDs) that were conducted on the basis of rational design
principles. The computed values of electronic transitions (λmax) in CDs molecules were
found in good agreement with the experimental measurements. From the experimental
results, it was found that the topical cyclohexenones exhibit intense violet fluorescence.
Moreover, computations of investigated molecules display high non-linear optical
response along with higher hyperpolarizability values. Extensive optical features of the
title compounds ultimately commend them as valuable moieties for the applications in
organic optoelectronic and light-emitting devices. In summary, the present investigations
allow a better quantitative understanding of the electronic polarization, underlying the
molecular NLO processes and the establishment of structure-property relationships.
5. Acknowledgment
18

The authors would like to extend their sincere appreciation to the Deanship of Scientific
Research at King Saud University for funding through the Research Group Project no
RGP-VPP-345.
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22

Synthesis, Characterization of Novel Cyclohexenone Derivatives and
Computation of their Optical Response
Graphical Abstract
1.0
500
400
300
200
100
0
hv
emission
1FuM4
1FuM2
1FuO
1FuF
1FuC4
NLO
absorption
O
O
0.8
0.6
0.4
0.2
0.0
O
O
R
R = H, Cl, OCH3,
R=
O
260
325
390
455
520
585
Wavelength (nm)
23

RESEARCH HIGHLIGHTS
Synthesis, Characterization of Novel Cyclohexenone Derivatives and
Computation of their Optical Response
•
•
•
•
A viable approach for the synthesis of five new cyclohexenone derivatives
from Robinson annulation is described.
Micro- and spectral analysis have been effectively operated to confirm the
molecular structures of cyclohexenones.
As-synthesized cyclohexenones exhibit intense violet fluorescence and
display high hyperpolarizability values.
The computed and experimental electronic transitions are in good agreement.
24